1. Field of the Invention
The present invention relates to a square lithium secondary battery.
2. Description of the Related Art
With recent advances in environmental technology, power generation devices (e.g., solar power generation devices) which pose less burden on the environment than in conventional power generation methods have been actively developed. Concurrently with the development of power generation technology, development of power storage devices such as lithium secondary batteries, lithium-ion capacitors, and air cells has also been advanced.
In particular, demand for lithium secondary batteries has rapidly grown with the development of the semiconductor industry, for the purpose of use in the next generation of clean energy vehicles such as hybrid electric vehicles (HEV), electric vehicles (EV), and plug-in hybrid electric vehicles (PHEV); personal digital assistants such as cellular phones, smart phones, and laptop computers; and portable consumer equipment such as portable music players and digital cameras. Now, the lithium secondary batteries are indispensable as a rechargeable energy source in the modern information society. In particular, square lithium secondary batteries with high capacity and high output are required for use in the electronic vehicles or the consumer equipment.
A square lithium secondary battery has a wound body obtained by winding a long sheet plural times, the long sheet including a positive electrode, a separator, and a negative electrode which overlap each other. Therefore, bends occurring in winding are likely to be broken, that is, an active material mixture layer on a corner is likely to be cracked, flaked, slipped, or the like so that the separated active material penetrates the separator, which might cause a short-circuit between the positive and negative electrodes. Thus, in Patent Documents 1 and 2, a wound body is formed in such a manner that an active material mixture layer is partly removed or flaked in advance so as not to be formed in a bend with a small radius of curvature.
A negative electrode used in a winding lithium secondary battery is manufactured by forming a negative electrode active material mixture layer on both surfaces of a sheet-like current collector. The material conventionally used as the negative electrode active material is graphite, which allows occlusion and release of ions serving as carriers (hereinafter, referred to as carrier ions). That is, graphite as a negative electrode active material, carbon black as a conductive additive, and a resin as a binder are kneaded to form a slurry, and the slurry is applied on a current collector and dried, whereby a negative electrode is manufactured.
On the other hand, in the case where silicon or phosphorus-doped silicon is used as the negative electrode active material, about four times larger amount of carrier ions can be occluded than that in the case of using carbon, and a silicon negative electrode has a theoretical capacity as high as 4200 mAh/g which is significantly higher than the theoretical capacity of a carbon (graphite) negative electrode, 372 mAh/g. Thus, silicon is a suitable material for increasing the capacity of a secondary battery, and a lithium secondary battery using silicon as a negative electrode active material is now being actively developed for the purpose of increase in capacity.
However, with an increase in the amount of occlusion of carrier ions, the volume of an active material greatly changes in accordance with occlusion and release of carrier ions in charge/discharge cycle, resulting in lower adhesion between a current collector and silicon and degradation in battery characteristics due to charge/discharge. Furthermore, in some cases, there is a serious problem in that silicon is damaged to be exfoliated or pulverized leading to loss of function as a battery.
Thus, in Patent Document 3 for example, a columnar or powder layer of microcrystalline or amorphous silicon is formed as a negative electrode active material layer over a current collector made of copper foil or the like with a rough surface, and a layer of a carbon material such as graphite having a lower conductivity than silicon is formed over the silicon active material layer. With such a structure, current can be collected through the layer made of the carbon material such as graphite even when the silicon active material layer is flaked, whereby degradation in battery characteristics can be reduced.